The auction houses and individual you cited are known for good, genuine antique flags. You are correct about eBay, though. While you can get decent antique flags at reasonable prices, you need to be an expert alredy. While most reproductions are quite obviously new flags, there are major reasons to be cautious about others: 1) new "decorative reproductions" that 'look old and feel old' that are being described as vintage flags by unscrupulous individuals, and 2) actual antique flags that are not as old as the seller claims (ie "Civil War era" or "Centennial Era" flags where the stars are sewn on with zig-zag stitching, a method patented for flags in 1892.)

To answer your question, finally, Jeffery Kenneth Kohn of Pennsylvania is a noted flag expert, generally dealing with larger flags. He worked with Sothebys auction gallery to produce the book "American flags: designs for a young nation", which is linked below. Problem is, I don't have contact info for Kohn.

If you are looking for Civil War battle flags, enlist the help of one or two trusted dealers. 150-year old museum pieces don't come out of attics very frequently these days.

Just what sort of flag(s) are you looking for?

Nick